Symantec said in a blog post that CrySys , the Hungarian research firm that discovered the Duqu Trojan earlier this month, has identified a dropper file that was used to infect systems with the malware.

The installer file is a malicious Microsoft Word document designed to exploit a zero-day code execution vulnerability in the Windows kernel.

"When the file is opened, malicious code executes and installs the main Duqu binaries" on the compromised system, Symantec said.

According to Symantec, the malicious Word document in the recovered installer appears to have been specifically crafted for the targeted organization. The file was designed to ensure that Duqu would only be installed during a specific eight-day window in August, Symantec noted.

No known workarounds exist for the zero-day vulnerability that Duqu exploits. The installer that was recovered is one of several that may have been used to spread the Trojan.

It is possible that other methods of infection are also being used to spread Duqu, Symantec noted.

Jerry Bryant, Microsoft's Trustworthy Computing group manager, said that the company is working "diligently" to address the issue.

"Microsoft is collaborating with our partners to provide protections for a vulnerability used in targeted attempts to infect computers with the Duqu malware," Bryant said in an email.

The company will issue a security update to address the vulnerability "through our security bulletin process," Bryant said.